1. Field of the Invention
The present invention relates generally to disk drive systems including an apparatus for positioning a flying head over a disk.
2. Related Art
Extremely high density data storage systems based on magneto-optic and optical storage disk media store data in at least one track including a series of very small regions, each region termed either a mark or a space. Marks are regions altered by a writing process and spaces are the regions between marks. The values of one or more bits are encoded as the lengths of the marks and spaces, depending upon the encoding technology. The track may be 0.2 .mu.m wide, with marks and spaces being one or more times that length. Because very small regions of this storage medium are used to represent information, a transducer system which can discriminate those regions with a high degree of resolution is used so that each bit can be accessed separately. If a single track is used, it forms a spiral with a close spacing or pitch between windings. If multiple tracks are used, they are also arranged with a close spacing or pitch therebetween. Because of the close spacing between the multiple track or track windings, high storage density requires accurate positioning of the transducer relative to the pitch of the track or tracks formed on the surface of the medium.
High density disk drive systems based on magneto-optic and optical storage principles generally use a transducer system which does not, under normal operating conditions, contact the surface of the recording medium. Some such non-contact transducers are known in this art as flying heads because of the principles upon which they rely to maintain a correct position with respect to the surface of the recording medium. A brief description of how a flying head flies is now given, with reference to FIG. 15.
During operation of a disk drive, the recording medium, typically in the form of a specially coated disk of aluminum, glass or plastic, rotates at high speeds, e.g., 3600 RPM. The rotary motion of the disk 107 causes an air flow in the direction of rotation, near the surface 106 of the disk 107. The head 101 is placed by a mechanical actuator or load arm 103 in proximity with the surface 106 of the disk so that the air flow passes between the surface of the disk and the lower features of the head, thereby forming a cushion of air 108 which generates an upwards force F.sub.A on the head 101 due to air pressure in the space between the disk surface and the lower features of the head 101, with the lower features of the head defining an air bearing surface 110. The cushion of air 108 that develops between the air bearing surface 110 and the surface 106 of the disk is referred to hereinafter as an air bearing.
The flying head 101 flies at a flying height 113, defined herein as the separation distance between the air bearing surface 110 of the head 101 and the surface 106 of the disk, determined by the force balance between the air pressure F.sub.A of the air bearing 108 pushing the head 101 away from the surface 106 of the disk, and a downward force F.sub.L exerted through a spring 105 or suspension that mounts the head 101 to the load arm or actuator 103.
The force F.sub.L has a magnitude determined by the physical dimensions of the spring, the spring constant of the spring material and the deformation of the spring which occurs in operation. The upward force F.sub.A applied by the air bearing depends on the finish of the disk surface, the linear velocity of the disk surface where it passes under the head, and the shape and size of the air bearing surface of the head. Whenever F.sub.A and F.sub.L are not equal, the head experiences a net force which causes it to move in a vertical direction corresponding to the direction of the net force. When F.sub.L =F.sub.A, the head experiences no net force, and hence no vertical motion.
In conventional systems, as flying height 113 increases, the air bearing 108 grows, lowering F.sub.A, while spring 105 is compressed, raising F.sub.L. The relationship between each of the forces F.sub.L and F.sub.A and flying height 113 can be determined by application of aerodynamic principles to the system configuration, which can be done by making measurements on actual systems, or physical or computer-generated models of the system. The conventional system is designed so that F.sub.L =F.sub.A at the desired flying height when the disk 107 is spinning at its normal speed. When the disk spins down, i.e., slows to a stop, insufficient air flow occurs to maintain the air bearing between the head and disk. Hence, insufficient air pressure and force are generated to counteract the downward force exerted by the spring or suspension, leading to contact between the head and disk. Thus, when the disk 107 slows to a stop, the head 101 may come to rest on the disk surface 106. Alternatively, the disk drive may include a mechanism that lifts the suspension 103 to prevent contact between the head and disk when the disk spins down, but otherwise plays no role in normal disk drive operation.
Flying height 113 is one important parameter governing successful operation of a disk drive. At extremely large values for flying height 113, excessive distance from the disk can cause unacceptable functional performance, for example, an inability to discriminate high frequency signals. Close proximity of the head to the disk improves functional performance. However, at extremely small values for flying height 113, insufficient flying height or loss of separation between the head and the disk can result in aerodynamic instability, reliability problems and catastrophic product failure, e.g., a head crash which occurs when the head contacts the disk surface with sufficient force to cause damage to the head or the disk surface resulting in a loss of data. Avoiding potential damage often associated with contact between the head and disk is the reason that some disk drives move their heads away from the disk surface to avoid contact when the disk spins down. The lowest height at which the head can fly without making contact with the disk surface is defined as the minimum glide height for the disk. Asperities (i.e., microscopic bumps or roughness) in the disk surface are those features which are likely to be contacted first by the head.
One problem of disk drive manufacturing is that the physical parameters determinative of flying height, e.g., the spring characteristics (affecting load force), the design of the air bearing surface shape, manufacturing variations in the air bearing surface geometry and finish (affecting air bearing force), and the load arm position relative to the surface of the disk (affecting load force), exhibit some variation within a tolerance band which causes a corresponding variation in the load force or air bearing force and, in turn, flying height. Other sources of variation in flying height in a disk drive include variations in altitude (i.e., ambient air density), radial position of the head on the disk which varies the velocity of the air flow due to different track circumferential lengths at different track radii, and skew angle of the head relative to a line tangential to a track, all of which affect the air bearing force.
Conventionally, flying height is set by a mechanical adjustment made at the time of manufacture of a disk drive. The mechanical adjustment sets a static load force selected to provide a desired flying height under nominal conditions. For example, the static load force may be measured manually and adjusted by repositioning or bending the load arm 103. Once set, the static load force remains substantially unaltered for the life of the disk drive, despite variations in operating conditions which may cause variation in other parameters determinative of flying height. Conventional systems are also known which employ closed loop feedback control systems to maintain a substantially constant flying height. Although such systems can compensate for variations in some parameters, there remain other uncompensated tolerance errors, such as variation in the actual minimum glide height from one disk to another.
Thus, flying height in conventional disk drives cannot be set to the minimum glide height. Rather, tolerance variations such as discussed above are typically taken into consideration, adding a tolerance band to the nominal or design minimum glide height of a disk when setting the actual flying height. Therefore, in order to avoid any likelihood of unwanted contact between the head and the surface of the disk, conventional systems set a nominal flying height that is greater than the largest expected actual minimum glide height. Conventional systems use this tolerance band because they have no way of determining the actual minimum glide height for the disk.
In view of the foregoing, one problem encountered in the prior art is that conventional flying head systems are unable to fly at the actual minimum glide height for a disk. By failing to fly at the minimum glide height, conventional systems exhibit poorer resolution than that of which they are theoretically capable. Moreover, in systems in which energy is transferred to the optical disk by an evanescent wave emitted from an optical element such as a lens through which an optical signal is passed, the energy transferred to the optical disk may be less than theoretically achievable. Therefore, in order to maintain an adequate signal at the disk surface, a more powerful laser might be required in such a conventional system.
Another problem with conventional optical disk drive systems is described making reference to FIG. 20, which illustrates an optical flying head and related disk drive components. The optical flying head 2001 includes a body 2003 having a lower surface defining an air bearing surface 2005. The head 2001 also has a two-element lens 2007 affixed to the body 2003. A laser light source 2009 emits a collimated laser beam 2011 which is passed through the lens, focused onto the recording medium surface 2013 and returned through a beam splitter 2015 to a detector 2017. When properly focused, the beam is caused to converge exactly at the recording medium surface. The focal length of the lens, i.e., the distance at which the collimated laser beam will converge after passing through the lens, is fixed. This distance depends on the lens element shapes and sizes. It is desired to minimize the spot size of the focused beam in order to maximize data storage density on the disk surface. Conventional systems are designed by those skilled in this art to use a diffraction limited lens system at the wavelength of the laser beam emitted by the laser light source and numerical aperture of interest, i.e., one in which the wavefront error is minimized by correctly setting the distance between lens elements for the anticipated flying height. If the beam fails to converge exactly at the recording medium surface due to flying height variation or other tolerances, then the density at which information can be recorded or recovered may be adversely affected.
In view of the foregoing, another problem encountered in conventional flying head systems is controlling the height of the lens above the disk so that it remains at the focal length of the lens. Since conventional systems fixedly mount the lens to the head, variations in flying height of the head necessarily result in variations in the height of the lens above the disk, which can result in degraded performance if that height is not at the focal length of the lens. In addition, variations in other characteristics of the system, including the positioning of the lens relative to the air bearing surface and the shape of the lens, can also affect whether focus is achieved. Conventional systems cannot independently compensate for these variations, because the height of the lens above the disk can be controlled only by varying the height of the head to which the lens is fixed.
As anyone with a conventional portable compact disk (CD) player knows, even the relatively coarse positioner of such a system is extremely susceptible to externally applied mechanical shocks. As briefly mentioned at the beginning of this specification, yet another problem of conventional systems is that the head must be positioned with an extremely fine positional resolution to properly discriminate between closely spaced adjacent tracks. However, a requirement for extremely fine positional resolution renders the system more susceptible to mechanical shock and vibration.
Another problem of conventional systems is to provide very fast positioning over a wide range of track positions, while also providing extremely high track position resolution to discriminate between closely spaced tracks. One conventional solution to this problem is to provide both a coarse positioner and a fine positioner which cooperate to position the head at the proper location. A conventional coarse positioner can quickly move the head to an approximate position defined by any track or group of tracks on a disk, but cannot accurately position to or follow the track on which reading and writing are to take place with sufficient accuracy. Therefore, once roughly positioned by the coarse positioner, the head is more finely positioned by the fine positioner. The fine positioner conventionally has a range of movement covering a distance equal to the span of distance occupied by a small group of tracks, or less, but extremely fine resolution. A problem with this conventional arrangement is that each time the coarse positioner operates to move the head by several tracks to a new track group, the movement has a similar effect upon the fine positioner as an external mechanical shock. That is, the coarse positioner adds to the final position error which the fine positioner will overcome, a transient error due to an induced mechanical shock. When the coarse positioner ceases its movement, the fine positioner must then overcome the induced mechanical shock, as well as position accurately over the target track before reading or writing of data can commence. This movement of the fine positioner takes longer to accomplish than movement of the fine positioner over the same distance would take if made without a movement of the coarse positioner to a different track group because the fine positioner requires additional time to overcome the induced mechanical shock caused by the coarse positioner movement.
In the optical disk drive industry are two distinct types of head/media systems currently known. One type of head/media system is that which is commonly employed in CD players. It uses a non-flying head and substrate incident media. The other type of head/media system uses a flying head and air incident media. These two head/media systems are seen as intrinsically incompatible.
Flying heads can be used to obtain higher storage capacity than non-flying heads. The flying head uses an air bearing to maintain a substantially fixed separation distance from the disk or the media, and hence a fixed focus. This air bearing requires a clean environment in which to operate as the air film separating the head from the disk can be as thin as 1 microinch. The head should never touch the disk. Dirt on the head or disk may cause a head crash. This head must also have very low mass optics in order to fly effectively, and in most cases the optics will be focused near the surface on which the head is flying. This is referred to as air incident.
Conversely, a non-flying head requires a focus actuator to focus the optics with respect to the media. Typically, this head focuses through a transparent substrate to a recording layer of the media in Digital Video Disk (DVD) or CD technology. In both cases this is referred to as substrate incident. Substrate incident disks are fairly immune to dirt because the light is not focused on the exposed surface, but rather a larger cross-section of light passes through the transparent substrate and converges to focus on the recording layer on the opposite side of the substrate. Thus, any dirt on the disk surface obscures but a fraction of the light focused through the disk.
The path the converting beam takes through the disk alters the light waves as they refract through the surface. This refraction causes spherical aberration of the focused beam. In contrast, the focussed beam in the air incident case does not exhibit spherical aberration. However, a spherical aberration correction can be built into the design of the optical elements. This correction, then requires that the light pass through a substrate. The optical elements all tend to have apertures &gt;3 mm in diameter so that the desired working distances to the media can be maintained.
Each type of system--flying and non-flying has its advantages. The non-flying head system is more rugged with well established industry standards. The flying head system, however, obtains a much higher recording density.
Conventional magneto-optic systems did use a two-head system in some instances. The purpose was to achieve direct-overwrite. Density was not improved. However, this implementation was in a sense the worst implementation because it had neither the robustness of substrate incident performance and compatibility nor the density gain associated with an optical flying head approach.
Conventional systems do not mix head technologies such as flying heads with remotely positioned non-flying heads such as used in CD players. These technologies are generally thought to be incompatible with each other. However, as disk types proliferate, this incompatibility results in a proliferation of corresponding disk drive systems to record and reproduce data.
It is an object of the present invention to provide an improved disk drive system.